System and method for radio transmitter acquisition

ABSTRACT

A method of a receiver determining the timing of a signal transmitted in a time-slotted manner, the signal comprising a sequence of information which is repeated at a known interval and has at least a known minimum length. The method performs correlation operations between groups of received slots of information, the groups spaced by the known interval. The groups are moved through the received signal, adding and removing slots, to locate a maximum correlation value sum for the group which should correspond to the timing of the slot. The method also can be used to determine a frequency offset at the receiver and/or an initial phase.

FIELD OF THE INVENTION

The present invention relates to system and method for a receiver toacquire a transmitter of a radio signal. More specifically, the presentinvention relates to a system and method for a radio receiver to acquireand interpret a multiplexed radio signal from a transmitter whichemploys a slotted, or other time-based, transmission structure.

BACKGROUND OF THE INVENTION

Advanced radio communications systems are being developed and deployedto provide wireless voice and data services. One example of such anadvanced radio communications system presently being developed is thatspecified by the Third Generation Partnership Project, or 3GPP, which isan international partnership of telecommunications standardsorganizations. More information about the communications systems beingspecified by the 3GPP, including technical specifications, can be foundon their web page www.3gpp.org, or from the various memberorganizations.

The proposed 3GPP system is a communications system which employscellular-type networks to permit communications between fixed basestation transceivers and customer transceivers which can be mobile orfixed. One of tasks such an advanced communications system must performis the acquisition of a base station transmitter by the receiver in acustomer device transceiver when the customer device is powered on andat various other times, for example to support handoff of the customerdevice between base stations. Acquisition of the base stationtransmitter by the customer device receiver includes many of the stepsrequired for communication to commence between the base station and thecustomer device, including determining synchronization/timing, carrieroffset and the initial phase of signals received by the customer devicefrom the base station.

Due to the complexity of the structure and arrangement of the physicalcommunication channels, multipath effects, etc., acquisition can bedifficult and/or computationally expensive to achieve. As will beapparent to those of skill in the art, this difficulty and/or complexitycan increase the cost of customer devices and/or can result in poorservice, for example if acquisition requires too long a time to achieve.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel method andsystem for a radio receiver to acquire a radio transmitter in acommunication system, the method and/or system obviating or mitigatingat least some of the above-identified disadvantages of the prior art.

According to a first aspect of the present invention, there is provideda method for a radio receiver to acquire timing information for a radiotransmitter which transmits, in a time-slotted arrangement, at least onesignal which includes a sequence of information indicating the slottiming and having a length at least equal to a known minimum length andwhich sequence is repeated in said at least one signal at a knowninterval, the method comprising the steps of:

(i) receiving a number of slots of said at least one signal which is atleast sufficient to allow reception of two repetitions of said knownminimum length of said sequence;

(ii) forming a first group of said received slots at least equal innumber to said known minimum length and a counterpart group of the samenumber of received slots but spaced from said first group by said knowninterval, each group having a first slot and a last slot;

(iii) for each slot in said first group and its corresponding slot inthe counterpart group, performing a correlation operation to obtain acorrelation value for information in each of these corresponding pairsof slots;

(iv) summing said obtained correlation values to obtain a correlationsum and storing said obtained correlation sum;

(v) obtaining a next correlation sum by:

-   -   (a) performing a correlation operation to obtain a correlation        value for information in the slot at the first slot of said        first group and the corresponding slot in said counterpart        group;    -   (b) performing a correlation operation to obtain a correlation        value for information in the slot outside the first group        adjacent to the last slot of said group and the corresponding        slot outside said last slot of said counterpart group;    -   (c) reforming said first group and said counterpart group to        exclude the respective slots first slots and to include the        respective slots adjacent the last slots; and    -   (d) from the correlation sum last obtained, subtracting the        correlation value obtained in (a) and adding the correlation        value obtained in (b) to obtain a correlation sum;

(vi) storing said correlation sum obtained in (d);

(vii) repeating steps (v) and (vi) until a number of correlation sumsequal to said known minimum length are obtained and stored;

(viii) examining said stored obtained correlation sums to select the sumwith the greatest magnitude, this selected sum indicative of thepresence of said sequence and thereby indicating the slot timing.

According to another aspect of the present invention, there is provideda method of determining frequency offset in a radio receiver from asignal transmitted by a radio transmitter which transmits, in atime-slotted arrangement, at least one signal which includes a sequenceof information indicating the slot timing and having a length at leastequal to a known minimum length and which sequence is repeated in saidat least one signal at a known interval, comprising the steps of:

(a) receiving said signal and determining the slot timing of saidsignal;

(b) forming a vector comprising determined correlation values between aninstance of said minimum length of said known information as received bysaid receiver and said known information signal;

(c) repeating step (b) for additional received instances of said minimumlength of said known information to obtain a set of vectors;

(d) forming an inner product of said set of vectors to obtain a set ofobtained products;

(e) determining an average value of said obtained products anddetermining the arctangent of the average value;

(f) determining from the nominal frequency of interest and apredetermined maximum error in the receiver the values, if any, whichcan be added to the determined arctangent;

(g) from

${{\Delta\; f} = \frac{B}{2{\pi({interval})}T}},$where B=tan⁻¹ (average value)+2π (the values determined in step (f)) andinterval is the number of slots between the start of instances of thesignal, determining the possible frequency offsets Δf for each valuedetermined in step (f) and testing each determined value by applying itto said minimum length of said known information as received by saidreceiver and then correlating the resulting information with said knowninformation signal; and

(h) selecting the possible frequency offset with the best correlationvalue determined in step (g) as the frequency offset.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will now be described, byway of example only, with reference to the attached Figures, wherein:

FIG. 1 shows the radio channel frame and slot structure proposed by the3GPP organization and the arrangement of a synchronization signaltherein;

FIG. 2 shows a graphical representation of a correlation operation in aconventional acquisition method;

FIG. 3 shows a graphical representation of a correlation operation inaccordance with the present invention; and

FIG. 4 shows a plot of the absolute value of a correlation value versusthe received chip at which the correlation operation was performed.

DETAILED DESCRIPTION OF THE INVENTION

The 3GPP system, discussed above, includes a primary synchronizationchannel (PSCH) and a secondary synchronization channel (SSCH) which arebroadcast from each base station and which are used by each customerdevice to acquire the base station. In the 3GPP system, all channels(including these two synchronization channels) are broadcast in the formof slotted frames, with most channels having frames of ten milliseconds(10 ms) duration and wherein each frame includes fifteen slots.

Presently, the 3GPP system is generally contemplated as being based uponCDMA multiplexing techniques and the following discussion relates to aCDMA related embodiment of the present invention. However, as will beapparent to those of skill in the art, the present invention can beapplied to other multiplexing techniques including OFDM, FDMA, TDMA andcombinations of such techniques such as GSM.

As discussed in the 3GPP documentation, the PSCH is used by customerdevices to determine the timing of the slots within frames transmittedby a base station. A predefined data sequence, the primarysynchronization sequence, is transmitted in the slots and frames of thePSCH and this sequence has been defined and arranged such that customerdevices can determine the start time of the slots in the framestransmitted from the base station by determining the location of thissequence in a received set of chips (in a CDMA implementation).

Once the slot timing has been determined by a customer device from thePSCH, the SSCH is examined by the customer device to determine thetiming of the frames, i.e. the start time of each frame, and otherinformation, including scrambling codes used by the base station, etc.The acquisition and processing of the PSCH and SSCH channels isperformed at the start up of a customer device as it first acquires abase station and, in mobile systems at least, is performed on an ongoingbasis for adjacent cells to permit handoffs between cells as thecustomer device moves between service areas.

FIG. 1 shows a PSCH frame, from the 3GPP system. As shown, a frame 20includes fifteen slots 24 ₁ through 24 ₂₅. Each slot 24 _(i) includestwo-thousand, five-hundred and sixty (2,560) chips so that frame 20 hasa total of thirty-eight thousand, four hundred (38,400) chips. Thebroadcast duration of frame 20 is ten milliseconds, for a chip rate ofthree million, eight-hundred and forty thousand (3.84 million) chips persecond.

As indicated, the primary synchronization sequence 28 is broadcast inthe first two hundred and fifty six chips of each slot 24 _(i). As partof the acquisition of the signals transmitted by a base station, thecustomer device performs a correlation between the received signal andthe known two-hundred and fifty-six predefined chips of primarysynchronization data to determine the slot timing.

Conventionally, as shown in FIG. 2, this correlation is performed atevery chip c_(i) in a block of received data 32 and requires chip c_(i)and the two-hundred and fifty-five following received chips (chipsc_(i+1) to c_(i+255)) to be correlated with the two-hundred andfifty-six known values for the primary synchronization sequence 28 andthe results summed and compared to those obtained when starting at eachother chip c_(i). In fact, to ensure identification of the primarysynchronization sequence 28 with a high degree of confidence, thecorrelation operation is typically performed starting at each of chipsc₁ to c₂₅₆₀, thus requiring the processing of two-thousand eight-hundredand sixteen chips (2,816), which is equivalent to one and a tenth slotsof received data.

In FIG. 2, the first correlation operation (shown at the top of theFigure) is performed starting at chip c₁ and proceeding through chipc₂₅₆ with the two-hundred and fifty-six values of the primarysynchronization sequence 28. The second correlation operation (shownbelow the first) starts at chip c₂ and proceeds through chip c₂₅₇ withthe same two-hundred and fifty-six values of the primary synchronizationsequence 28. The correlation operation is performed for each subsequentstarting chip c_(i), up to the last operation, shown at the bottom ofthe Figure, which is performed starting on chip c₂₅₆₀ and proceedingthrough chip c₂₈₁₆ with the two-hundred and fifty-six values of theprimary synchronization sequence 28.

The highest absolute valued result of these operations, indicating thebest correlation, should be found at the location of the first receivedchip of the primary synchronization sequence 28, thus indicating thestart timing of the received slots.

When actually implemented, the received chips are typically at leastdouble-sampled and filtered with a square root raised cosine filter.Thus, the known copy of the primary synchronization sequence is also atleast double-sampled and filtered with a square root raised cosinefilter before the correlation is performed.

While this conventional technique does work, it requires a large amountof computation to be performed. Specifically, performing the correlationfor each one and a tenth slots requires six-hundred and fifty-fivethousand, three-hundred and sixty (655,360) complex multiplicationoperations (256 multiplications of complex numbers for each of 2816chips), multiplied by the sampling rate, which is typically at leasttwo, and the same number of addition operations. Thus, this techniquecan be both computationally expensive and time consuming.

The above-mentioned disadvantage of the conventional technique can befurther exacerbated if other signals broadcast by the transmitter beingacquired are not well behaved. Specifically, in the 3GPP system, alldata is transmitted based upon the ten millisecond frame, fifteen slot,structures discussed above. Thus, the customer device is performing thecorrelation on the total received signal which can include the signalstransmitted on other channels. In the 3GPP system, only QPSK (quadraturephase shift keying) modulation is employed for transmitted signals andthe power level of signals modulated with QPSK is not excessive withrespect to the power levels of the PSCH and SSCH signals. In suchcircumstances, acquisition can often be achieved after correlating oneor a few slots of received data.

However, if other signals are transmitted using QAM (quadratureamplitude modulation) modulation, as proposed by the assignee of thepresent invention, or other modulation techniques, or the signals areotherwise not well behaved, the power levels of transmitted signals onsome channels relative to those of the PSCH and SSCH may be large,requiring correlation to be performed over one or more frames ofreceived signal before sufficient confidence is obtained in the result.Correlating a single frame of received signal with the above-describedconventional acquisition technique requires over nine million complexnumber multiplication operations to be performed, as well as requiringlarge amounts of memory to store the correlation results for comparison.

The present inventor has determined that slot timing can be obtained ina more computationally, time and memory-efficient manner, relative tothe conventional acquisition technique, according to the followingsystem and method.

Unlike the conventional method discussed above, wherein the knownprimary synchronization sequence is correlated over at least an entireslot of the received signal, the present invention tales advantage ofthe fact that the primary synchronization sequence repeats after aknown, and fixed, number of chips. In the 3GPP system, the primarysynchronization sequence is transmitted in the first two-hundred andfifty-six chips of the two-thousand, five-hundred and sixty chips of aslot and the primary synchronization sequence is repeated every slot.Accordingly, the present invention determines the correlation between areceived chip and a counterpart chip received two-thousand, five-hundredand sixty chips (i.e.—one slot) before or after. Thus, the correlationvalue, Cor(k), at a received chip, r(k), is the sum of the two-hundredand fifty-six complex multiplications (multiplications of complexnumbers) of each of the received chips r(k) to r(k+256) with receivedcounterpart chips r(k+2560) to r(k+2560+256) or

${{Cor}(k)} = {\sum\limits_{i = k}^{k + 256}{{r(i)}*{{r( {i + 2560} )}.}}}$For the first iteration, Cor(k) is solved for k, where k is the firstreceived chip to be examined, and this requires two hundred and fiftysix complex multiplication operations and the same number of additionoperations to be performed.

Next, Cor(k+z), where z varies over the balance of a complete slot, i.e.1≦z≦2559, must be determined. However, as will be apparent, bycorrelating the received signal with an offset counterpart receivedsignal, a moving sum method can be employed. Unlike the conventionalmethod wherein two-hundred and fifty-six multiplication and additionoperations are required to be performed when the correlation is to beperformed for the next received chip, with the moving sum method of thepresent invention, Cor(k+z) is determined fromCor(k+z)=Cor(k+z−1)−(r(k+z−1)*r(k+z−1+2560))+(r(k+256+z)*r(k+256+z+2560))In other words, after any Cor(k+z−1) has been determined (and on thevery first iteration z=1 and Cor(k+z−1)=Cor(k)), the next correlationvalue Cor(k+z) can be obtained by subtracting from the value determinedfor Cor(k+z−1) the correlation value of the first received chip r(k+z−1)in that last determined value with its offset received chipr(k+z−1+2560) and determining and adding a correlation value for thenewly included received chip r(k+z+256) and its offset received chipr(k+2560+z+256).

Thus, the first determined correlation value Cor(k) requires two-hundredand fifty-six complex multiplication and addition operations and eachsubsequent Cor(k+z) requires two additional complex multiplications [oneto calculate r(k+z−1)*r(k+z−1+2560) to be subtracted and one tocalculate r(k+z+256)*r(k+1+2560+z+256) to be added], and one additionand one subtraction operation.

FIG. 3 illustrates this graphically. In the Figure, a number of receivedchips 40 has been obtained. The repeated sequence 42, has a length oftwo-hundred and fifty-six chips and is repeated every two-thousand andfifty-six chips.

A first received chip r₁, is selected arbitrarily, and all the rest ofthe received chips r_(i) are considered relative to this arbitrarystarting point. In other words, received chip r₂₅₆ is defined as havingbeen received two-hundred and fifty-five chips after received chip r₁and received chip r₂₅₆₂ is defined as having been received two-thousand,five-hundred and sixty-one chips after received chip r₁. In the Figure,one instance of repeated sequence 42 is shown commencing at receivedchip r₁₀₂₀ and the next instance is shown commencing at received chipr₃₅₈₀.

Received chips r₁ through r₂₅₆ are correlated against received chipsr₂₅₆₁ through r₂₈₁₆, respectively, to solve for Cor(1), with r₁ beingcorrelated with r₂₅₆₁, r₂ being correlated with r₂₅₆₂, etc. Next Cor(2)is solved for by taking the value determined for Cor(1) and subtractingthe contribution 44 of the correlation of r₁ and r₂₅₆₁, and adding thecontribution 48 of the correlation of r₂₅₇ and r₂₈₁₇. Typically, theprocess is repeated for at least the known interval over which thesignal is repeated, such as a slot. In the present example, wherein theprimary synchronization sequence repeats every two-thousand,five-hundred and sixty chips, at least Cor(1) through Cor(2560) aredetermined. FIG. 4 shows the result of the absolute value of Cor(1)through Cor(2560). As can been seen, the peak value occurs for Cor(1020)which is a start location of the repeated sequence.

Even if more than one slot length of signal must be processed, forsufficient confidence, the method merely continues subtracting andadding contributions from each previous and successive pair of chips.The present inventors refer to this as a fast scan acquisition method.

Thus, determining correlation values over an entire slot with the fastscan acquisition method requires much less computation that theconventional acquisition method described above, and this difference iseven greater if more than one slot must be processed. Further, while thediscussion above has been with reference to the presently proposed 3GPPsystem wherein a predefined synchronization sequence is employed, thefast scan acquisition method can also be employed in systems wherein thesynchronization sequence is not predefined (or known) to the customerdevice. Specifically, as long as the interval at which thesynchronization sequence is repeated and the sequence has at least aknown minimum length, the fast scan acquisition method can be employed.

Further, the sequence need not comprise a contiguous set of chips(i.e.—some number of adjacent chips) nor is the sequence limited to anyparticular number of chips (i.e.—two-hundred and fifty-six chips vs.three-hundred chips). The sequence can comprise any periodic sequence ofany desired length, as will be apparent to those of skill in the art,where the periodicity of the sequence and a minimum sequence length isknown by the receiver. For example, the synchronization sequence cancomprise: chips one through seventy-five; chips one-thousand throughone-thousand two-hundred; and chips two-thousand through two-thousandand seventy-five of a slot. The length will be selected to provide adesired level of confidence in the result, with longer sequencesgenerally providing greater levels of confidence.

It is contemplated that less than a full sequence can be processed by aparticular receiver. For example, the sequence can occupy the entireslot and one receiver may only consider one-tenth of the slot whileanother, which requires a greater degree of confidence in the result,can process one-half or even all of the sequence.

Many other sequences of suitable lengths and periodicities will beapparent to those of skill in the art and are limited only by the wellknown design requirements for such sequences, including the need for thesequence to generate an appropriate autocorrelation response and to havea sequence length long enough to provide sufficient confidence in thecorrelation result.

Depending upon operating conditions experienced by a customer device,and in particular for low received SNR's, the fast scan acquisitionmethod disclosed above may not afford sufficient confidence and/oraccuracy in the result. In such a case, the fast scan acquisition methodabove can be used to determine a best estimate of the location of thestart of the slots, k_(est) by identifying the peak absolute Cor( )value from the set of determined Cor(k)'s, i.e.—fromk _(est)=arg max (|Cor(k)|), 1≦k≦2560(the upper limit of k can exceed 2560 if more than one received slot wasprocessed). Once k_(est) has been determined, conventional-typeacquisition methods, as described above with respect to FIG. 2 or anyother suitable technique, can be employed over a selected number ofreceived chips preceding and following k_(est) refine the slot timing.For example, the region of interest can be deemed to extend fromk_(est)−40 to k_(est)+40 and the conventional acquisition techniquesdescribed above can be used on this interval for refinement.

If additional accuracy is required, perhaps due to very low SNR levels,etc., both the above-described fast scan and/or fast scan andconventional acquisition over a selected region of interest, can beperformed independently in the I and Q quadratures. Further, these stepscan be performed over several slots, an entire frame or even multipleframes until a desired level of accuracy and/or confidence is obtained.

An additional problem in acquiring a transmitter occurs in determiningthe frequency offset which is experienced at the receiver due tooscillator error at the receiver. For example, achieving oscillatoraccuracy of greater than three parts per million (3 ppm) is expensiveand many systems, for cost reasons, specify that a 5 ppm oscillator issufficient. However, the receiver must be able to determine thefrequency offset which results from this oscillator error in order tocorrectly receive signals.

If a transmitter transmits a signal s_(k), then in a sampled domain thereceiver will receiver(k)=ŝ(k)e ^(jΔf2πkT) e ^(Jφ) +n(k)where ŝ is the received version of the transmitted signal (resultingfrom multi-path effects), Δf is the frequency offset, φ is the initialphase, T is the duration of a chip and n is the noise. In a continuousdomain, the received signal isr(t)=ŝ(t)e ^(j2πft) e ^(jφ) n(t)

In the present invention, after slot timing has been determined, avector V is constructed of the first two hundred and fifty six chips ofeach slot in a received frame which are correlated with the knownprimary synchronization sequence (psc_(i)) to obtain vectors

${{V(1)} = \begin{pmatrix}{{psc}_{1}*r_{1}} \\\vdots \\{{psc}_{256}*r_{256}}\end{pmatrix}},{{V(2)} = \begin{pmatrix}{{psc}_{1}*r_{2560 + 1}} \\\vdots \\{{psc}_{256}*r_{2560 + 256}}\end{pmatrix}},\ldots\mspace{11mu},{{V( N_{acc} )} = \begin{pmatrix}{{psc}_{1}*r_{{({14*2560})} + 1}} \\\vdots \\{{psc}_{256}*r_{{({14 + 2560})} + 256}}\end{pmatrix}}$

As indicated above, it may be desired and/or required to consider morethan a frame of slots of received signal to achieve the desired level ofconfidence and/or accuracy. Thus, N_(acc) slots can be considered, whereN_(acc) can be greater than fifteen (or in other slot structures, lessthan fifteen). The frequency offset information is contained in vectorsV(k). One method to extract this information is described below, butother suitable methods will be apparent to those of skill in the art.

To obtain the frequency offset information, the inner product of V canbe determined to yield N_(acc)−1 data pointsα(m)=V(m)^(*T) V(m+1), m=1, 2, . . . , (N _(acc)−1)which are proportional to e^(j2π2560ΔfT). Then, defining

${\frac{1}{N_{acc}}{\sum\limits_{m = 1}^{N_{acc}}{\alpha(m)}}} = A$and2π2560ΔfT=∠(A)=Beverything needed to determine Δf is known, except B=2π2560Δf modulo 2π.However, as mentioned before, the oscillator in a receiver is typicallyspecified as having a known maximum error and the range of possiblevalues for B can easily be determined. For example, if the receiveroscillator is specified as having a maximum error of 5 ppm and if thetransmission frequency is 1.8 GHz, then B can only have thirteen values,specifically B=∠A+2π×{0, ±1, ±2, . . . , ±6}. As will be apparent tothose of skill in the art, if the primary synchronization sequence is adifferent number of chips and/or the number of chips per slot and/or themaximum oscillator error differ in other implementations, the aboveoperations will be modified appropriately.

Therefore, each of these thirteen possible values of B is evaluated byapplying the value B_(i) to the received primary synchronizationsequence 28 (in our example the first two-hundred and fifty-six chips ina received slot) and correlating the result to the known primarysynchronization sequence 28. The best correlation will occur with thecorrect value for B. Once B is known, the frequency offset, Δf, can bederived from

${\Delta\; f} = \frac{B}{2\;\pi\; 2560T}$as all of the other quantities are now known.

If it is desired to determine the initial phase φ, to initialize a RAKEreceiver for example, this can now also easily be determined. Thevectors V( ), described above, can also be used for this purpose. Ifκ=Σe^(jkTΔf2π), then φ can be determined from

${\mathbb{e}}^{j\phi} = {\sum\limits_{k = 1}^{N_{acc}}\frac{{sum}( {V(k)} )}{N_{acc}*\kappa}}$

As discussed above, the present invention can provide significantadvantages over prior art acquisition methods and systems by reducingcomputational complexity, memory requirements and the time required toacquire a radio transmitter. In addition, the method and system of thepresent invention can also be employed in other advantageous manners.

As an example of one such additional use of the present invention, if acustomer device employs a steerable antenna (either electrically ormechanically steerable) to receive signals from a base stationtransmitter, the fast scan acquisition method described above can beemployed to quickly determine an antenna direction with acceptablereception characteristics. For example, if an electrically steerableantenna with four possible directions is employed, each direction can beselected in turn and the fast scan acquisition method can be performedfor that direction and the results of the fast scan from each directioncan be used to select an acceptable direction for furthercommunications. In an embodiment of the present invention, themagnitudes of the peak correlation value determined for each directionare compared and the greatest magnitude direction is selected.

Another example of an additional use of the present invention is for acustomer device to monitor reception levels of other base stations, orbase station sectors (in the case of multi-sector base stations) topermit handoff of the customer device between base stations or sectors.In this context, the customer device can, on an intermittent basis,perform a fast scan for each base station, or base station sector, ofinterest to obtain an initial indication of the reception levels at thecustomer device for each transmitter. The customer device can use thisinformation to request hand-off from a present base station or basestation sector to another base station or base station sector which itcan receive at better levels, or this information can be transmitted tothe base station from the customer device, and then on to a networkmanagement system which can monitor and/or determine if a handoff shouldbe performed.

Other uses and advantages of the present invention will be apparent tothose of skill in the art. The above-described embodiments of theinvention are intended to be examples of the present invention andalterations and modifications may be effected thereto, by those of skillin the art, without departing from the scope of the invention which isdefined solely by the claims appended hereto.

1. A method for a radio receiver to acquire timing information for aradio transmitter which transmits, in a time-slotted arrangement, atleast one signal which includes a sequence of information indicating theslot timing and having a length at least equal to a known minimum lengthand which sequence is repeated in said at least one signal at a knowninterval, the method comprising the steps of: (i) receiving a number ofslots of said at least one signal which is at least sufficient to allowreception of two repetitions of said known minimum length of saidsequence; (ii) forming a first group of said received slots at leastequal in number to said known minimum length and a counterpart group ofthe same number of received slots but spaced from said first group bysaid known interval, each group having a first slot and a last slot;(iii) for each slot in said first group and its corresponding slot inthe counterpart group, performing a correlation operation to obtain acorrelation value for information in each of these corresponding pairsof slots; (iv) summing said obtained correlation values to obtain acorrelation sum and storing said obtained correlation sum; (v) obtaininga next correlation sum by: (a) performing a correlation operation toobtain a correlation value for information in the slot at the first slotof said first group and the corresponding slot in said counterpartgroup; (b) performing a correlation operation to obtain a correlationvalue for information in the slot outside the first group adjacent tothe last slot of said group and the corresponding slot outside said lastslot of said counterpart group; (c) reforming said first group and saidcounterpart group to exclude the respective slots first slots and toinclude the respective slots adjacent the last slots; and (d) from thecorrelation sum last obtained, subtracting the correlation valueobtained in (a) and adding the correlation value obtained in (b) toobtain a correlation sum; (vi) storing said correlation sum obtained in(d); (vii) repeating steps (v) and (vi) until at least a number ofcorrelation sums equal to said known minimum length are obtained andstored; (viii) examining said stored obtained correlation sums to selectthe sum with the greatest magnitude, this selected sum indicative of thepresence of said sequence and thereby indicating the slot timing.
 2. Themethod of claim 1 wherein said indication of slot timing in step (viii)is employed to identify a region of interest in said received number ofslots of said at least one signal for processing by a subsequentacquisition operation.
 3. The method of claim 1 wherein the sequence ofinformation indicating the slot timing is known to both the transmitterand receiver and, once said slot timing is determined, the frequencyoffset in said received is determined from the steps of: (1) forming avector comprising determined correlation values between said an instanceof said minimum length of said known information as received by saidreceiver and said known information signal; (2) repeating step (1) foradditional received instances of said minimum length of said knowninformation to obtain a set of vectors; (3) forming an inner product ofsaid set of vectors to obtain a set of obtained products; (4)determining an average value of said obtained products and determiningthe arctangent of the average value; (5) determining from the nominalfrequency of interest and a predetermined maximum error in the receiverthe values, if any, which can be added to the determined arctangent; (6)from ${{\Delta\; f} = \frac{B}{2{\pi({interval})}T}},$ whereB=tan⁻¹(average value)+2π (the values determined in step (5)) andinterval is the number of slots between the start of instances of thesignal, determining the possible frequency offsets Δf for each valuedetermined in step (5) and testing each determined value by applying itto said minimum length of said known information as received by saidreceiver and then correlating the resulting information with said knowninformation signal; and (7) selecting the possible frequency offset withthe best correlation value determined in step (6) as the frequencyoffset.
 4. The method of claim 1 wherein the sequence of informationindicating the slot timing is known to both the transmitter and receiverand, once said slot timing is determined, the initial phase offset atsaid receiver is determined from the steps of: (1) forming a vector Vcomprising determined correlation values between said instance of saidminimum length k of said known information as received by said receiverand said known information signal for N_(acc) instances of said receivedinformation; (2) determining the initial phase φ from${\mathbb{e}}^{j\phi} = {\sum\limits_{k = 1}^{N_{acc}}\frac{{sum}( {V(k)} )}{N_{acc}*{\sum{\mathbb{e}}^{j\;{kT}\;\Delta\;{f2}\;\pi}}}}$where T represents the duration of each signal and Δf represents thefrequency offset.
 5. A method of determining frequency offset in a radioreceiver from a signal transmitted by a radio transmitter whichtransmits, in a time-slotted arrangement, at least one signal whichincludes a sequence of information indicating the slot timing and havinga length at least equal to a known minimum length and which sequence isrepeated in said at least one signal at a known interval, comprising thesteps of: (a) receiving said signal and determining the slot timing ofsaid signal; (b) forming a vector comprising determined correlationvalues between an instance of said minimum length of said knowninformation as received by said receiver and said known informationsignal; (c) repeating step (b) for additional received instances of saidminimum length of said known information to obtain a set of vectors; (d)forming an inner product of said set of vectors to obtain a set ofobtained products; (e) determining an average value of said obtainedproducts and determining the arctangent of the average value; (f)determining from the nominal frequency of interest and a predeterminedmaximum error in the receiver the values, if any, which can be added tothe determined arctangent; (g) from${{\Delta\; f} = \frac{B}{2{\pi({interval})}T}},$ where B=tan⁻¹(averagevalue)+2π(the values determined in step (f)) and interval is the numberof slots between the start of instances of the signal, determining thepossible frequency offsets Δf for each value determined in step (f) andtesting each determined value by applying it to said minimum length ofsaid known information as received by said receiver and then correlatingthe resulting information with said known information signal; and (h)selecting the possible frequency offset with the best correlation valuedetermined in step (g) as the frequency offset.